Self-normalizing flow sensor and method for the same

ABSTRACT

An apparatus to normalize a flow rate of a fluid in a main flow channel is provided. The apparatus uses a moveable member, such as a flexible membrane disposed for reciprocating displacement, to produce a constant dither flow of the fluid that is independent of fluid composition. This dither flow generates a signal output from a normalizing flow sensor that both represents a characteristic property of the fluid and a flow rate calibration factor. A similar apparatus to determine the characteristic property or flow rate calibration factor is also provided. The devices disclosed may be used in numerous industrial, process, and medical flow system applications for normalization of flow sensors and to derive other properties of a fluid.

This Application claims priority from U.S. Provisional Application Ser.No. 60/137,464, filed Jun. 4, 1999, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to flow sensors and, moreparticularly, to self-normalizing flow sensors and methods of usethereof.

2. Description of Related Technology

Sensors have been used to measure flow rates in various medical,process, and industrial applications, ranging from portable ventilatorssupplying anesthetizing agents to large-scale processing plants in achemical plant. In these applications, flow control is an inherentaspect of proper operation which is achieved in part by using flowsensors to measure the flow rate of a fluid within the flow system. Inmany flow systems, e.g., fuel gas flow systems containing a binarymixture of methanol and water, the chemical composition of the fluid maychange frequently. Also, a flow system is often required to flow morethan one fluid having different chemical and thermophysical properties.For example, in a semiconductor processing system that passes a N₂ gas,the N₂ gas may at times be replaced by a H₂ or He gas, depending on theneeds of the process; or in a natural gas metering system, thecomposition of the natural gas may change due to non-uniformconcentration profiles of the gas.

Measuring the flow rates of fluids of differing chemical compositions,i.e., differing in density, thermal conductivity, specific heat, etc.,requires calibrations of the flow sensor. Without recalibration, theflow sensor could produce accurate flow rate measurements for one fluidbut not another. Typically, flow sensors are calibrated upon theirinitial operation and, as such, are calibrated to compute accurate flowrate values only for fluids with a particular, narrow range of chemicalcomposition.

Known ways of re-calibrating a flow sensor, or providing a calibratedflow rate measurement, do exist. In some instances, customers provideflow sensor manufacturers with information on the composition of eachfluid to be measured by the flow sensor. From this information,manufactures perform calibration tests and obtain data for use in makingcalibration functions, or look-up tables, from which the flow ratesensor can be calibrated. In other instances, previously installed flowsensors will be taken off-line so that a re-calibration for a new fluidcan be performed. This process essentially reinitializes a flow sensorfor accurate measurement with respect to fluids having differingchemical compositions and, therefore, is a costly and inconvenientoption for customers.

An on-line calibration technique had been developed using a propertysensor, i.e., a thermal sensor, to measure the specific heat and thermalconductivity properties of a fluid and a flow sensor to measure anuncalibrated flow rate. The specific heat and thermal conductivity(along with absolute temperature and Prandtl No.) are related to a flowrate correction factor, C_(V), by a known equation. Therefore, theseproperty sensors, connected to a flow channel through a dead-endrecessed cavity into which fluid enters principally by diffusion,measure values which must then be applied to expensive andtime-consuming computational circuitry or microprocessors before thecorrection factor, C_(V), and subsequent calibrated flow signal can beproduced.

The calibration correction factor, C_(V), is related to the calibratedflow rate by the following expression: V_(c)=V_(u)/C_(V), where V_(u) isthe uncalibrated flow rate measured by a flow sensor in the main flowchannel and V_(c) is the calibrated flow rate. Using C_(V) is helpfulbecause the correction factor is not dependent of the flow rate, andonly depends on the chemical composition and properties of the fluid,primarily the thermal conductivity and specific heat of the fluid.

The flow sensor measuring the uncalibrated flow rate, is typically athermal anemometer, which measures the difference in temperature betweenupstream and downstream sensing elements by measuring the differences inresistance between each sensor. Relative temperature changes between thetwo sensors result from convection effects that occur due to the flow ofthe fluid passing by the heated elements. From this difference, the rawor “uncalibrated” flow sensor signal and flow rate of a fluid can bederived. If only a single fluid is to be measured, then one could setthe C_(V)=1 assuming the sensor had been initially calibrated to measurethis fluid. However, if other fluid compositions are to be measured,then the determination of C_(V) for these fluids is required.

This known technique of measuring thermal conductivity and specificheat, calculating a correction factor based on these values, and thenderiving a normalized flow rate is costly and slow due to the complexityof the data processing involved. The relationship between C_(V),specific heat, and thermal conductivity requires substantial computingpower to derive the former from the latter two. Slow response time is aparticular problem in many applications in which the composition of thefluid may change naturally from minute to minute, a condition thatfrequently occurs in flow systems in which the fluid is natural gas,gasoline, fuel oil, or other chemical binary, tertiary, or quaternarymixtures.

The present invention is directed to a flow sensor which can beself-normalized to produce accurate flow rate measurements for varyingtypes of fluids, both liquid and gaseous, and to be able to produce suchmeasurements at an affordable cost and with a relatively short responsetime.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, an apparatus for use innormalizing a main flow rate of a fluid in a main flow channel isdisclosed. The apparatus comprises a normalizing flow sensor and amoveable member. The normalizing flow sensor measures a dither flow rateof the fluid as the fluid. The moveable member is disposed for producingthe dither flow rate. The dither flow rate is substantially independentof the flow rate in the main flow channel and is substantiallyindependent of the fluid composition.

In accordance with another aspect of the invention, a self-normalizingflow sensor apparatus for use in measuring a main flow rate of a fluidin a main flow channel is disclosed. The apparatus comprises a main flowsensor, a normalization flow sensor, and a moveable member. The mainflow sensor measures the flow rate of the fluid in the main flowchannel. The normalization flow sensor measures a dither flow rate ofthe fluid in response to a dither flow of the fluid. A dither flow rateof the dither flow is substantially independent of the flow rate of thefluid in the main flow channel. The moveable member is disposed forreciprocating movement so as to produce the dither flow of the fluid.

In accordance with yet another aspect of the invention, a normalizingflow sensor comprising a flexible membrane disposed for reciprocatingmovement is disclosed. The flexible membrane produces a dither flow of afluid near said flow senor, such that the dither flow is substantiallyindependent of fluid composition. The normalizing flow sensor measuresthe dither flow rate of said fluid.

In accordance with still another aspect of the invention, a method ofnormalizing a flow sensor that measures a flow rate of a fluid in a flowchannel comprises the following steps: receiving a portion of the fluidfrom the flow channel; moving a membrane according to known displacementto create a dither flow of the received portion; measuring a dither flowrate of said dithered received portion; and communicating said ditherflow rate to a processor which uses the dither flow rate to compute anormalized flow rate.

In accordance with a further aspect of the invention, a method ofmeasuring a chemical component in a fluid flowing in a flow systemcomprises the following steps: receiving a portion of said fluid fromsaid flow channel; creating a dither flow of said received portion;measuring a dither flow rate of said dithered fluid; and determining theconcentration in a binary mixture based upon the measured dither flowrate.

In accordance with a further aspect of the invention, a method ofdetermining a higher value property of a fluid, the method comprisingthe flowing steps: receiving a portion of said fluid from said flowchannel; creating a dither flow of said received portion; measuring adither flow rate of said dithered fluid; and determining said highervalue property based upon the measured dither flow rate and othermeasured properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the general concept of the presentinvention showing a flow rate sensor that measures an uncalibrated flowrate, a dither flow rate-producing membrane, and a normalizing sensorfor measuring a calibration value derived from the dither flow rate.

FIG. 2 is a three-dimensional view of the exterior of a flow sensornormalizing apparatus formed of modular flow channel and dither channelsections according to an embodiment of the invention.

FIG. 3 is a cross-sectional top view of the flow channel section of FIG.2 (cut along 3 and looking in the direction of the arrows) showing aflow rate sensor disposed in a sensing channel connected to a flowchannel.

FIG. 4 is a cross-sectional bottom view of the dither channel section ofFIG. 2 (cut along 4 and looking in the direction of the arrows) showinga dither membrane in a dither chamber disposed above the flow channel ofFIG. 2. A normalizing sensor in connection to the dither chamber is alsoshown.

FIG. 5 is a cross-sectional side view of the flow sensor normalizingapparatus of FIG. 2 (cut along 5 and looking in the direction of thearrows) showing a cross-sectional view of both the flow channel sectionand the dither channel section.

FIG. 6 is an expanded and detailed view of an exemplary actuator thatreciprocally moves the dither membrane according to an embodiment of thepresent invention.

FIG. 7 is an exemplary block diagram of the control circuitry of FIG. 1which computes the calibrated flow rate.

FIG. 8 is a graph of the calibration correction factor, C_(V), measuredby the normalizing sensor of the FIGS. 2-5 as a function of drivefrequency of the dither membrane and input voltage for N₂ gas for twolevels of input voltage.

FIG. 9 is a graph of the measured ΔG values for the main flow sensor andthe normalizing flow sensor for a flow rate at standard temperature andpressure.

FIG. 10 is a graph of the computed ΔG_(RMS) as a function of drivefrequency for fluids of different composition.

FIG. 11 is a graph of the purge times of the apparatus of FIG. 2 at twoflow rates with a bleed hole open and closed.

FIG. 12 is a graph of the uncalibrated flow rates measured by the flowsensor for different fluids and the calibration factor, ΔG_(cal)=C_(V),measured by the normalizing sensor for the same fluids.

FIG. 13 is side view of an alternative three sensor embodiment in whicha dither flow perturbation is measured from a flow sensor disposedupstream of a main flow sensor and another sensor disposed downstream ofthe main sensor.

FIG. 14 is a graph of measured properties of a fluid, including C_(V)values, as a function of mole concentration of methanol vapor in watervapor.

FIG. 15 is a graph of measured properties of a fluid, including C_(V)values, as a function mole concentration of ethane in methane gas.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To overcome the disadvantages of the prior art discussed above, a firstself-normalizing flow sensor device 30 that directly measures thecalibration, or correction factor C_(V), for normalizing the measuredflow rate of a main flow sensor 32 is provided in FIG. 1. As will beapparent from the following description, the apparatuses disclosed belowmay be used in numerous in medical, process, and industrialapplications. Furthermore, it will be apparent from the description thatthe calibration factor may also be used to derive other properties of afluid, such as the concentration of a chemical composition in a binarymixture fluid, or “higher value” properties (compressibility factor,density, viscosity, heating value, oxygen demand, octane number, andcetane number). In fact, numerous types of implementations of theinvention will be apparent from the description and claims that follow.These implementations are considered with the scope of the presentinvention.

The first self-normalizing flow sensor device 30 of FIG. 1 is a generaldepiction of an embodiment of the invention. The first self-normalizingflow sensor device 30 includes a main flow channel 34 through which afluid, gaseous or liquid, will travel. The main flow channel 34 iscompatible for use with existing flow systems, and as such is shaped ina circular cross-sectional shape of uniform cross-sectional sizethroughout the length of the flow channel 34. The main flow channel 34,however, can be of varying size, that is of smaller or largercross-sectional size at certain locations to alter the flow rate rangecapability accordingly. Though not shown, it will be appreciated bypersons of ordinary skill in the art that the flow channel 34 can beterminated with inlet and outlet mounts, such as threadable mounts, foraffixing the first self-normalizing flow sensor device 30 into such anexisting flow system. The preferred direction of fluid flow in the flowchannel 34 is shown by arrows.

In operation, the first self-normalizing flow sensor device 30determines a calibrated flow rate. Therefore, disposed along an upperwall of the flow channel 34 is the flow sensor 32 for measuring the flowrate of the fluid traveling through the flow channel 34. The flow sensor32 can be one of numerous types of flow sensors, such as optical flowsensors, orifice-based flow sensors, delta-pressure sensors across anorifice or across a laminar flow restriction, and Pitot tubes, but ispreferably a hot-element thermal anemometer in the form of a microbridgeflow sensor, such as the AWM43300 made by the Micro Switch Division ofHoneywell Inc., which has the advantage of accurate measurement andprolonged lifetime in comparison to other flow sensors that are moresusceptible to damage and contamination effects. This off-the-shelfthermal anemometer is also inexpensive. The flow sensor 32 isexemplarily shown connected to control circuitry 36 via an electricalconnection 38 and an electrical connection 39 both of which may comprisemultiple leads, as shown by example in FIG. 2.

The first self-normalizing flow sensor device 30 also comprises anormalizing flow sensor 40 disposed in a dither chamber 42 connected tothe flow channel 34 via a sensing tap 44 formed of a single opening inthe main flow channel 34. The normalizing flow sensor 40 can beidentical to the flow sensor 32, and, therefore, as with the main flowsensor 32, the normalizing flow sensor 40 can take any number of forms,including those listed above with respect to the flow sensor 32, but ispreferably a microbridge flow sensor. Preferably, the main flow sensor32 is identical to the normalizing flow sensor 40. The signal measuredby the normalizing flow sensor 40 is used to calibrate the signalmeasured by the flow sensor 32. The dither chamber 42 defines a dithersensing channel 46 to which the normalizing flow sensor 40 is connected.Under pressure equalization operating conditions, a portion of the fluidflowing in the flow channel 34 diffuses into the dither chamber 42through the sensing tap 44, with the sensing tap 44 being sized so thatthe fluid in the dither chamber 42 is substantially free of anyturbulence effects occurring in the flow channel 34. The sensing tap 44provides equalization between the flow channel 34 and the dither channel46 so that no compositional, temperature, or pressure gradients existbetween the two channels. Because the normalizing flow sensor 40 is usedto normalize the measurements of the flow sensor 32, the normalizingflow sensor 40 must measure the same fluid that is being measured by theflow sensor 32, i.e., a fluid of identical thermophysical properties. Anoptional exchange wall 45 can also be disposed in the dither sensingchannel 46 and extending to the tap 44 to direct ingress and egress ofthe fluid diffusing into the dither channel 46.

In normalizing the output of the flow sensor 32, the normalizing flowsensor 40 measures the calibration factor, C_(V), which is related tophysical properties, such as density, specific heat and thermalconductivity, of the fluid. This correction factor is communicated tothe control circuitry 36 via lead 48, so that the control circuitry 36can perform the calculation necessary to derive the calibrated flowrate, i.e, C_(c)=V_(u)/C_(V), where V_(u) is the uncalibrated flow rateand V_(c) is the calibrated flow rate. Moreover, the conditions in thedither chamber 42 and sensing channel 46 should be such that what ismeasured by the normalizing flow sensor 40 is substantially independentof flow rate. To achieve this condition, a dither membrane 50 isdisposed for vibratory movement in the dither chamber 42. In the exampleof FIG. 1, the dither chamber 42 defines the sensing channel 46, and thedither membrane 50 displaces a constant volume (or mass under specialconditions) in a periodic or reciprocating manner generatingintermittent forward and reverse flow rates in the dither sensingchannel 46. The preferred direction of the dither flow is shown byarrows. The dither flow created by the membrane 50 is of constantmagnitude and repeating, i.e., the displaced volume or mass issubstantially without variation per stroke and substantially independentof time and fluid properties, such as pressure, temperature, and fluidcomposition. Fluid composition refers to the chemical make-up of thefluid. Combining the effect of the dither membrane 50 with thenon-positive displacement normalizing flow sensor 40, it is apparentthat what is disclosed by the invention is a heretofore non-existapparatus for creating a positive displacement flow sensor device from anon-positive displacement flow sensor combined with a dither membrane ofknown volumetric displacement.

A driver 52 drives the dither membrane 50. The driver 52 may be amomentum driven mechanism, thermal expansion driven mechanism,pistonically driven pump, piezo-electric driven pump, electromagneticdriven pump, or other suitable device. The driver 52 and the membrane 50form the actuator 53. To reduce costs, the actuator 53 is preferably anearphone speaker 54 (See FIG. 6) driven by an sinusoidal or triangularwave input signal that produces constant (AC sinusoidal or square-wave,respectively) periodic motion in the membrane 50. In FIG. 1, an actuatoris generally shown as a magnetic coil 56 and a bobbin 58 disposed formoving the membrane 50.

As the membrane 50 vibrates, a dither flow rate of the fluid, that issubstantially independent of the flow rate of the fluid in the flowchannel 34 is produced and measured by the flow sensor 40. Thepeak-to-peak displacement of the reciprocating membrane 50 can occurover a wide range so long as it is constant and substantiallyindependent of the factors identified above.

For laminarizing the flow of the fluid in the flow channel 34, twohoneycomb flow restrictors 60 a, 60 b are placed in the flow channel 34upstream and downstream of the flow sensor 32, respectively. These flowrestrictors reduce turbulence within the flow channel 34 so that lessnoise is measured and less output signal fluctuation occurs at the flowsensor 32. Laminarizing the flow in this way also reduces the presenceof turbulence effects in the dither sensing channel 46.

The exterior of a second self-normalizing flow sensor device 62 is shownin FIG. 2. The second self-normalizing flow sensor device 62 differsprincipally from that of FIG. 1 in that the flow sensor 32 is disposedin a bypass channel 64 parallel to the flow channel 34 (FIG. 3). Thesecond self-normnalizing flow sensor device 62 is shown formed of twosections, a flow channel section 66 and a dither channel section 68.This multiple section depiction, however, is done to better convey theoperation of the second self-normalizing flow sensor device 62, andshould not be construed as limiting the ways in which the secondself-normalizing flow sensor device 62 or the more general firstself-normalizing flow sensor device 30 may be implemented. Either thefirst self-normalizing flow sensor device 30 or the secondself-normalizing flow sensor device 62 could be formed of a singlesection or multiple sections and still be within the scope of theinvention.

A flow tube 70, also provided with honeycomb flow straighteners, definesthe flow channel 34 of section 66, while a bleed valve 72 and anactuator I/O connector 74 are shown extending from the section 68. Threeleads of the flow sensor 32 (not shown), an output lead 76, an inputlead 78, and a ground 80, are disposed on the exterior of the secondself-normalizing flow sensor device 62 for attaching the flow sensor 32to the external control circuitry 36. For example, the input lead 78 cansupply +10 V to the heater for raising it to the typical 160° C.operating temperature. In a similar manner, three identical leads at therear surface of the second self-normalizing flow sensor device 62connect to the normalizing flow sensor 40. The bleed valve 72 isoptional, though preferred. The bleed valve 72 is a small rod connectedto open and close a bleed hole 82, or purge hole, in the dither chamber42 (FIG. 4).

A cross-sectional view of the section 66 is shown in FIG. 3. The bypasschannel 64 connects to the flow channel 34 at an inlet 84 and a outlet86. The inlet 84 and outlet 86 are of a circular cross-sectional shape.O-rings 88 a, 88 b maintain a substantially air-tight, affixedconnection at the inlet 84 and the outlet 86, respectively. TheseO-rings 88 a, 88 b are employed because the preferred embodiment usesoff-the-shelf sensors which must be affixed to the flow channel 34 toform the second self-normalizing flow sensor device 62. However,depending on design, they may not be advantageous.

The operation of the flow sensor 32 in this bypass configuration issimilar to that of FIG. 1, in that both measure the flow rate of aportion of the fluid flowing past a flow sensor. The bypassconfiguration is useful, however, because it reduces the likelihood ofthe flow sensor 32 being damaged by high velocity fluid flow orcontamination which can occur to sensors disposed within the flowchannel 34. Furthermore, the bypass configuration allows high flow ratesthrough the flow channel 34 because only a portion of this flowing fluidpasses through the sensing channel 64. The second self-normalizing flowsensor device 62, for example, can be operated accurately at flow ratesin excess of 200 ltrs/min. Alternatively, with the flow sensor 32disposed in the flow channel 34, as in FIG. 1, only flow rates on theorder of 1 ltr/min can be accurately measured before the flow sensor 32becomes saturated.

Two ports, a dither inlet 90 a and a bleed outlet 90 b, connecting theflow channel 34 to the dither sensing channel 46, are shown in phantomand can meet the flow channel 34 at O-ring connections like those atinlet 84 and outlet 86 of the sensing channel 64. The dither inlet 90 aacts as a port, via flow segments extending perpendicularly from FIG. 3,to the same identified port of FIG. 4 which in turn leads to the sensingchannel 46. A portion of the fluid in the flow channel 34 diffuses intothe sensing channel 46 via inlet 90 a , through a zero-sum exchange. Thebleed outlet 90 b is similarly disposed as shown in FIGS. 3 and 4, andalso connects to the sensing channel 46. The bleed outlet 90 b differs,however, from that of the dither inlet 90 a in that the bleed outlet 90b is completely capped to prevent ingress and egress of the fluid to theflow channel 34 except through the small bleed hole 82 which is openedand closed by the bleed valve 72. The ports 90 a, 90 b define the ditherchannel 46 housing the normalization sensor 40.

In the open position, the bleed hole 82 purges the dither chamber 42 tothe flow channel 34, thus, leaving the dither sensing channel 46 with asmall bypass outlet to the flow channel 34. This purging is beneficialwhen the second self-normalizing flow sensor device 62 is going tomeasure flow rate of a fluid of different thermophysical properties thanthe fluid it had most recently been measuring. When the bleed hole 82 isin the closed position, the dither sensing channel 46 is a completelydead-end cavity.

In FIG. 4, the dither membrane 50 is disposed at one end of the actuator53, which is operated via two leads 94, 96 connecting to the actuatorI/O connector 74. With the actuator 53 being formed by the earphonespeaker 54 (FIG. 6), for example, the two leads 94, 96 could beconnected to an AC signal source for driving the membrane at frequenciesbetween 10 to 700 Hz, with a preferred range of linear operation of thesecond self-normalizing flow sensor device 62 in the 40 to 100 Hz range.

FIG. 5 shows a cross-sectional view of both the section 66 and thesection 68 when combined for operation. The view of FIG. 5 is orthogonalto the views of FIGS. 3 and 4.

Referring to FIG. 6, a circular steel diaphragm 98 of the earphonespeaker 54 acts as the dithering membrane 50. The two leads 94, 96extend from the actuator I/O connector 74 to a wound coil bobbin 100. Aswill be appreciated by persons of ordinary skill in the art, an ACsignal applied to the bobbin 100 produces an oscillating electromagneticfield through magnets 104, 106 and the bobbin 100 which will cause thediaphragm 98 to vibrate in a reciprocating, pistonic motion. Thediaphragm 98, and thus the membrane 50, can be driven over a wide rangeof frequencies. However, in this preferred embodiment that range extendsbelow the resonance, or eigen, frequencies of the membrane, magnets, andother speaker components, i.e., approximately 2000 Hz. Opposite ends ofthe diaphragm 98 of FIG. 6, or diaphragm 50 of FIGS. 4 or 5, areconnected via channels 46 to the normalizing flow sensor 40.

An exemplary block diagram of the control circuitry 36 is provided inFIG. 7. The flow sensor 32 is connected to an A/D converter block 120which receives an analog signal representing the measured uncalibratedflow rate and communicates a corresponding digital signal to amicroprocessor block 122. The correction factor is derived from theoutput measured by the normalizing flow sensor 40 at a synchronousdemodulator block 126 communicating therewith, is communicated to themicroprocessor block 122 via converter block 120, as well. Thesynchronous demodulator block 126 demodulates the AC signal from thenormalizing flow sensor 40 by amplifying a modulation frequency equal tothe actuator drive frequency output by an electronic actuator driver128, thus reducing the bandwidth of the noise of the AC signal. Thedemodulator block 126 outputs an RMS signal to the AD converter block120. The electronic actuator driver block 128 functions as an electronicsignal generator, providing a constant frequency and constant voltageoutput to the actuator 53 via the actuator I/O connector 74. Inaddition, the electronic actuator driver block 128, communicates dataregarding the input signal to the actuator 53, such as the drivingfrequency to the synchronous demodulator block 126, for rectifying orcreating the RMS output of the normalizing flow sensor 40, this signalbeing the correction factor, C_(V).

The microprocessor block 122 is programmed to compute the normalizedflow rate based on the received data and can be programmed to performnumerous other operations on the data such as deriving a compositionconcentration from the correction factor as in FIGS. 13 and 14. Thecontrol circuitry 36 may exist in a single controller connected to thesecond self-normalizing flow sensor device 62 or may be implemented bymultiple controllers individually connected together and any number ofthese controllers could be incorporated into the second self-normalizingflow sensor device 62 through known manufacturing techniques. Thecontrol circuitry 36 can output the calibrated flow rate to an RS232port for display to an operator.

Though the above is preferred, alternatively the control circuitry 36can be slightly modified into a flow sensor calibrating arrangement withfeedback loop configuration eliminating the need for the microprocessor122. The demodulated correction factor sent from the demodulator 126 tothe AID converter 120 could be sent directly to a Wheatstone bridgecontaining the flow sensor 32, the Wheatstone bridge being used tomeasure the resistance values of the downstream and upstream sensors ofthe flow sensor 32 for producing the measured flow rate output. The A/Dconverter 120 would convert the received analog correction factor signalinto an RMS DC signal for connecting to the Wheatstone bridge. In thiscase, the output from the flow sensor 32 would have been internallycalibrated as a result of the feedback loop, leaving no furthercalibration computation necessary.

An advantage of the invention is that the first self-normalizing flowsensor device 30 or the second self-normalizing flow sensor device 62functions or on-line flow. In the on-line mode, the dither membrane 50and normalizing flow sensor 40 are used to compute the calibrationfactor, C_(V). The correction factor, C_(V), is equal to either thepeak-to-peak square-wave or sine-wave RMS output of the normalizing flowsensor 40, i.e., ΔG_(cal, RMS). Exemplary ranges of linearity andnon-linearity are shown in FIG. 8, which also shows the effect ofdifferent input voltages, peak-to-peak, on the ΔG output. Thus, tonormalize or calibrate the flow signal provided by the flow sensor 32,only the value C_(V) is derived from the output of the flow sensor 40,eliminating the need for time consuming computations which result fromknown devices which only measured thermal conductivity and specificheat. Given known formulations for expressing C_(V) as a function ofthermal conductivity, specific heat, Prandtl No., and absolutetemperature, expected C_(v) values for a few gases are: He=0.139759;Ar=0.966175; CH₄=0.971325; N₂=0.998816; C₂H₆=2.27872; and C₃H₈=3.9979.

Exemplary Operation of a Self-normalizing Flow Sensor Device

In one example of the implementation of the second self-normalizing flowsensor device 62, the ΔG measured as a function of the standard flowrate, Vs (L/h), of a N₂ gas at 24.4° C. and 993 mbars of pressure isshown in FIG. 9. For this measurement, the flow channel 34 had an innerdiameter of 12.5 mm, and the flow sensors 34, 40 were both AWM43300microbridge flow sensors. Furthermore, three honeycomb flowstraighteners of ⅛″ cell size were disposed in the flow channel 34. Thebleed hole 82 had an inner diameter of 0.25 mm. With the bleed hole 82closed by the bleed valve 72, however, the measured ΔG values are moreaccurate because the bleed-hole shunt flow shown by the square points inFIG. 9 do no reduce the flow sensed by the flow sensor 32. As shown bythe measured data of the normalizing flow sensor output with the bleedhole open, the effect of the bleed hole in the open position on the flowrate of the fluid in the sensing channel 46 is negligible except belowabout 10 L/h and above about 600 L/h of flow in the main channel.

The input wave forms to the actuator 53 and the correspondingnormalizing flow sensor 40 were sinusoidal wave forms of betweenapproximately 10 Hz to 850 Hz. Though a triangular wave can be used todrive the actuator 53, it was found that a sinusoid wave form producedbetter signal-to-noise ratios at the output, and therefore is preferred.FIG. 8 demonstrates that for N₂ gas a region of linearity over thisdrive frequency range was found to exist between about 10 Hz to 100 Hz.FIG. 10 shows a similar range of linearity for propane.

FIG. 10 shows the ΔG_(RMS) output of the normalizing flow sensor 40 as afunction of the drive frequency of the actuator 53 for two differentgases. Given a 3 Vpp input sine wave, a 4 to 40 L/h RMS signal wasmeasured for ΔG_(RMS) between 10 and 100 Hz drive frequency.

To determine the responsiveness of the system under differentconditions, the purge response times for changes in gaseous fluidcomposition as the second self-normalizing flow sensor device 62 wasswitched from measuring He to N₂ and C₃H₈ to He were measured. Responsetimes of approximately 60 seconds to 100 seconds were measured. Duringthese measurements, the bleed hole 82 was closed by the bleed valve 72.Response times for a change in gas composition from Ar to N₂ with thebleed hole 82 open are shown in FIG. 11, from which it is apparent thatlonger response times result when the bleed hole is open. Furthermore,data measured with the bleed hole 82 closed resulted in more accuratemeasures with better signal-to-noise ratios. The bleed hole 82 ispreferably left in the open position only during purging of the flowchannel.

FIG. 12 is similar to FIG. 9 except that it shows measurements of boththe flow sensor 32 and the normalization sensor 40 for three differentgases. The horizontal nature of the ΔG output for the normalizationsensor 40 confirms that ΔG is substantially independent of flow rate forflow rates below approximately 4000 L/h.

FIG. 13 shows an alternative embodiment of the invention in which athree sensor arrangement is used. As with the embodiments describedabove, the flow sensor 32 is disposed to measure the flow rate of afluid traveling in the flow channel 34. Like FIG. 1, the flow sensor 32is disposed within the flow channel 34 and not in a bypass channel. Inparallel with the flow channel, the dither membrane 50 is in the dithersensing channel 46. Though still amounting to only relatively smallperturbations in comparison to the flow rate of the fluid in the flowchannel 34, the dither flow created by the dither membrane 50 will alterthe flow rate of the fluid in the main channel upstream and downstreamof the flow sensor 32, and at flow sensor 32, because of the two portbypass arrangement. Thus, an upstream flow sensor 150 and a downstreamflow sensor 152 are positioned within the flow channel 34. Both flowsensors 150, 152 are ostensibly the same as flow sensor 32 in type and,as such, measure a flow rate of the fluid. The measured values fromthese two flow sensors 150, 152 are communicated to control circuitry154 which calibrates the measurement of the flow sensor 32 bydetermining the difference between the two measured values. Thisdifference being attributable primarily to the affect of thereciprocating dither membrane 50.

In addition to using the measured ΔG value to calibrate the flow sensor,as the graphs in FIGS. 14 and 15 show, the measured C_(V) value can alsobe used to characterize a binary-composition fluid and determine itsmole composition, without the need of the flow sensor 32. In theexamples of FIGS. 14 and 15, the value of the C_(V) signal issubstantially linear and substantially the strongest over the entirerange of concentrations from 0% to 100%, whereas many other parameters,except for mole weight, demonstrate nonlinear characteristics over thisregion. Therefore, the invention discloses that the measurement of C_(V)can be used to determine composition of mixtures, primarily binarymixtures at an affordable cost.

Moreover, having now derived a simple approach to determine correctionfactor C_(V) it will readily apparent to those of ordinary skill in theart that the above embodiments can be used to derive higher valueproperties of fluids via correlations between C_(V) and other measurableproperties (thermal conductivity and specific heat). Among these highvalue properties are properties such as compressibility factor, density,viscosity, heating value, oxygen demand, octane number, and cetanenumber. Because the C_(V), thermal conductivity, and specific valuecorrelates to each of these based on known functional relationships,such relationships would preferably be polynomial to allow the use ofinexpensive computational microprocessors which can produce outputs in arelatively short time. However, the known log, exponential or otherfunctional relationships could also be used and would be readilyapparent.

Those of ordinary skill in the art will appreciate that, although theteachings of the invention have been illustrated in connection withcertain embodiments, there is no intent to limit the invention to suchembodiments. On the contrary, the intention of this patent is to coverall modifications and embodiments fairly falling within the scope of theappended claims either literally or under the doctrine of equivalents.

What We claim is:
 1. An apparatus for use in normalizing a main flowrate of a fluid in a main flow channel, comprising: a normalizing flowsensor that measures a dither flow rate; and a moveable member disposedfor producing said dither flow rate, said dither flow rate beingsubstantially independent of said main flow rate in said main flowchannel and substantially independent of fluid composition.
 2. Theapparatus of claim 1, further comprising a main flow sensor and aprocessing unit, wherein the main flow sensor measures said main flowrate in the main flow channel, and wherein the processing unit isresponsive to said normalizing flow sensor and said main flow sensor toderive a normalized flow rate based on said measured dither flow rateand said measured main flow rate.
 3. The apparatus of claim 2, whereinsaid main flow sensor is disposed in a bypass channel communicating withsaid main flow channel.
 4. The apparatus of claim 2, wherein said flowsensor is disposed in said main flow channel.
 5. The apparatus of claim1 wherein the moveable member is arranged to produce reciprocatingmovement of a constant displacement, and wherein said dither flow rateis established by said reciprocating movement.
 6. The apparatus of claim5, wherein said moveable member is a flexible membrane driven by anelectrical actuator in the form of a miniature speaker.
 7. The apparatusof claim 5, wherein the moveable member is driven by a mechanicalvolumetric pump.
 8. The apparatus of claim 5, wherein the moveablemember is driven by a piezo-electric or electromagnetic actuator.
 9. Theapparatus of claim 1, wherein the normalizing sensor is a non-positivedisplacement flow sensor.
 10. The apparatus of claim 1, wherein saidnormalizing flow sensor and said moveable member are disposed in adither sensing channel that communicates with said main flow channel viaa sensing tap through which a portion of said fluid from the main flowchannel can exchange fluid mass with said dither sensing channel. 11.The apparatus of claim 10, further comprising a bleed hole extendingfrom said dither sensing channel to said main flow channel to purge saidportion of said fluid from said dither sensing channel.
 12. Theapparatus of claim 11, wherein said bleed hole is closeable.
 13. Aself-normalizing flow sensor apparatus for use in measuring a main flowrate of a fluid in a main flow channel, comprising: a main flow sensormeasuring the flow rate of said fluid in said main flow channel; anormalization flow sensor measuring a dither flow rate of the fluid inresponse to a dither flow of the fluid, where said dither flow rate ofsaid dither flow is substantially independent of the main flow rate ofthe fluid in the main flow channel; and a moveable member disposed forreciprocating movement so as to produce the dither flow of the fluid.14. The self-normalizing apparatus of claim 13, further comprising aprocessing unit responsive to said normalizing flow sensor and said mainflow sensor to derive a calibrated normalized main flow rate based onsaid measured dither flow rate and said main flow rate in said main flowchannel.
 15. The self-normalizing apparatus of claim 13, wherein theflow sensor is disposed in said flow channel to measure the main flowrate of said fluid in said main flow channel.
 16. The self-normalizingapparatus of claim 13, further comprising a dither sensing channel,wherein said normalizing flow sensor and said moveable member aredisposed in the dither sensing channel, and wherein said dither sensingchannel communicates with said main flow channel via a sensing tapthrough which a portion of said fluid in the flow channel diffuses. 17.The self-normalizing apparatus of claim 16, further comprising a bleedhole from said dither sensing channel to said main flow channel toaccelerate purging of fluid from said dither sensing channel.
 18. Theself-normalizing apparatus of claim 17, wherein said bleed hole iscloseable.
 19. The self-normalizing apparatus of claim 13, wherein thenormalizing flow sensor is a non-positive displacement flow sensor. 20.The self-normalizing apparatus of claim 13, wherein said main flowsensor is disposed in a bypass channel communicating with said main flowchannel.
 21. The self-normalizing apparatus of claim 13, wherein saidmain flow sensor is disposed in said main flow channel.
 22. Anormalizing flow sensor, comprising; a flexible membrane disposed forreciprocating movement and producing a dither flow of a fluid near saidflow sensor, said dither flow being substantially independent of fluidcomposition, such that said flow sensor measures a dither flow rate ofsaid fluid.
 23. A method of normalizing a flow sensor that measures aflow rate of a fluid in a flow channel, the method comprising the stepsof, receiving a portion of said fluid from said flow channel; moving amembrane according to known displacement to create a dither flow of saidreceived portion; measuring a dither flow rate of said dithered receivedportion; and communicating said dither flow rate to a processor whichuses the dither flow rate to compute a normalized flow rate.
 24. Themethod of claim 23, wherein the step of measuring said dither flow ratecomprises the step of providing a normalizing flow sensor in a channelcontaining said received fluid portion to measure said dither flow rateof said received portion.
 25. The method of claim 23, wherein said fluidis a binary mixture.